This disclosure relates to maintaining print quality in xerographic developer systems. More particularly, the teachings herein are directed to apparatus and methods for loading one or more donor rolls in a developer system.
Generally, the process of electrophotographic printing includes charging a photoconductive member such as a photoconductive belt or drum to a substantially uniform potential to sensitize the photoconductive surface thereof. The charged portion of the photoconductive surface is exposed to a light image from a scanning laser beam, a light emitting diode (LED) source, or other light source. This records an electrostatic latent image on the photoconductive surface. After the electrostatic latent image is recorded on the photoconductive surface, the latent image is developed in a developer system with charged toner. The toner powder image is subsequently transferred to a copy sheet and heated to permanently fuse it to the copy sheet.
The electrophotographic marking process given above can be modified to produce color images. One electrographic marking process, called image-on-image (IOI) processing, superimposes toner powder images of different color toners onto a photoreceptor prior to the transfer on the composite toner powder image onto a substrate, such as paper. While the IOI process provides certain benefits, such as a compact architecture, there are several challenges to its successful implementation. For instance, the viability of printing system concepts, such as IOI processing, require developer systems that do not interact with previously toned images.
In the developer system, two-component and single-component developer materials are commonly used. A typical two-component developer material comprises magnetic carrier granules having toner particles adhering triboelectrically thereto. A single-component developer material typically comprises toner particles. Since several known developer systems such as conventional two component magnetic brush development and single component jumping development interact with the photoconductive surface, a previously toned image will be scavenged by subsequent developer stations if interacting developer systems are used. Thus, for the IOI process, there is a need for a scavengeless or noninteractive developer systems such as the Hybrid Scavengeless Development (HSD).
In scavengeless developer systems such as HSD, developer materials are maintained in a reservoir and conveyed onto the surface of a conventional magnetic brush roll, also referred to as a mag roll, based on a magnetic field necessary to load the mag roll. Toner is conveyed from the surface of the mag roll onto the donor roll. The donor roll is held at an electrical potential difference relative to the mag roll to produce the field necessary to load toner from the surface of the mag roll onto the surface of the donor roll. The toner layer on the donor roll is then disturbed by electric fields from a wire or set of wires to produce and sustain an agitated cloud of toner particles, which are attracted to the latent image to form a toner powder image on the photoconductive surface.
Current embodiments of scavengeless developer systems use a single mag roll to load two donor rolls. There are many shortfalls associated with this current method of loading donor rolls.
One area of concern is the effective life of the developer materials. The use of developer materials beyond the effective life can be exhibited by the persistent appearance of print quality defects such as streaks. As developer ages, highly charged toner fines accumulate on the wires and cause the print quality defects.
Developer material aging has been observed to correlate with wire pollution voltage. A comparison of wire pollution voltage versus developer age demonstrates a “developer crash” behavior that is observed where the wire pollution voltage under sustained low area coverage printing increases suddenly as the developer ages. This problem is currently being managed with the injection of fresh toner into the developer housing, which has been shown to stabilize print quality performance. Another countermeasure is periodically cleaning the wires electrostatically against a bare donor roll. The resort to such measures would not be needed, or would be needed on a less frequent basis, if developer systems and methods were implemented to prolong the effective life of developer materials.
It has been demonstrated that developer material aging is a strong function of mag roll rotational speed. Operating at a slower mag roll speed improves developer life, and correspondingly, faster mag roll speeds are detrimental to developer life.
Donor roll loading systems typically utilize a trim blade, also referred to as a metering blade or a trim, to remove excess developer material from a mag roll to ensure an even depth of coverage with developer material before arrival at a first donor roll loading nip. In these systems, further improvements in developer life can be achieved by trimming at a slower mag roll rotational speed. It is widely believed that much of the material abuse in a developer housing happens in the trim region. Simulations of the elastic energy distribution in a development housing during operation demonstrate that the trim region is a high stress zone. The material abuse rate is proportional to the speed of the mag roll at the trim region. The material abuse rate can be minimized, and developer life extended, by operating the mag roll in which developer material is trimmed at slow rotational speeds.
Although lowering mag roll speed improves print quality with respect to the problem of developer material aging, excessively slow mag roll speeds are detrimental to print quality because of insufficient reload. Reload is the requirement to provide a sufficient supply of toner, via the mag roll, to the donor loading nip. The donor loading nip is the zone in which toner is delivered from the mag roll onto the donor roll. The optimal mag roll speed is dictated by a balance between slowing down the mag roll rotational speed to extend developer material life and speeding up the mag roll rotational speed to meet the threshold requirements of reload.
An additional problem associated with print quality performance is mottle. Mottle occurs when there is poor developer material transfer efficiency, either between the mag rolls and the donor rolls (wherein toner is transferred at the donor loading nips) or between the donor roll and the photoconductive belt (wherein toner is transferred at the developer nips). The rotational speed and direction of rotation of the donor roll rotation influences mottle. More specifically, mottle is influenced by the rotational direction of the donor roll in relation to the transport direction of the photoconductive belt, as well as in relation to the rotational direction of the mag roll. As shown in FIGS. 1 and 2, current scavengeless developer systems operate in the “against directional mode,” in which the mag roll rotates in a direction that is “against” the direction in which the donor roll rotates. In addition, the current scavengeless developer systems operate in the “same directional mode,” in which the donor roll rotates in the “same” direction as the direction of the photoconductive belt. It has been shown that this configuration is the worst from the point of view of mottle. In contrast, significant improvements in mottle have been demonstrated using the combination of the “with directional mode,” in which the mag roll rotates in a direction that is “with” the direction in which the donor roll rotates; and the “opposite directional mode,” in which the donor roll rotates in the “opposite” direction from the transport direction of the photoconductive belt.
Current scavengeless developer systems provide limited operational flexibility in simultaneously addressing the competing problems of developer life, reload and mottle to maintain acceptable levels of print quality.
There is a need for new scavengeless developer systems and methods of operating developer systems that can optimize print quality with respect to the problems of developer life, reload and mottle; at higher print speeds than are currently attainable. It is unlikely that current scavengeless developer systems can meet ambitious goals set for improved developer life and image quality improvements with respect to reload and mottle for speedup demanded in the market.
There has been provided an apparatus for loading one or more donor rolls of a developer unit, including: a developer housing having a reservoir for a developer material, the developer material comprising toner, a rotatable first donor roll that delivers the toner onto a moving photoconductive member, a rotatable trim roll that receives the developer material from the reservoir and delivers the developer material to a first magnetic brush roll that delivers the toner to the first donor roll, and a controller, responsive to a reload sensitivity signal, for controlling the speed of the trim roll.
While specific embodiments are described, it will be understood that they are not intended to be limiting. For example, even though the example given is a color process employing Image-On-Image technology, the disclosure is applicable to any system having donor rolls that are loaded by a magnetic brush, such as monochrome systems using just DC or AC/DC voltages to develop toner to the photoreceptor.
These and other objects, advantages and salient features are described in or apparent from the following detailed description of exemplary embodiments.